3850: Set COS, keep DSCP

[Alex Passfall]

Hello,

Is it possible to have a 3850 set the COS bits on a frame but keep the DSCP values in the IP packet?

According to http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/qos/configuration_guide/b_qos_3se_3850_cg/b_qos_3se_3850_cg_chapter_011.html#concept_359473EBF99C489697FB146963ED74D5 COS is set to 0 for DSCP tagged IP packets that leave on a trunk port.

Since the uplink switch in my case is a Nexus 5548 that will only look at the COS value for bridged packets, I want to set the COS. However, when I manually set it in the input policy-map, the DSCP value is also set accordingly (CS5 instead of EF for COS 5).

I found that I can use table-maps to modify the mappings, but I can only apply the table-map to class-default. I don't see how I could apply this in my case.

Currently, my QoS configuration looks somewhat like this:

class-map match-any VOICE
  match  dscp ef
class-map match-any SIGNALING
  match  dscp af31
  match  dscp cs3

policy-map INPUT-QoS
 class VOICE
    set cos 5
    police cir 100000000
      conform-action transmit
      exceed-action drop

 class SIGNALING
    set cos 4
    police cir 10000000
      conform-action transmit
      exceed-action drop


policy-map OUTPUT-VOICE-UPLINK
 class VOICE
     priority level 1
     police rate 100000000
 class SIGNALING
    bandwidth remaining percent 20
    queue-buffers ratio 20
 class class-default
    bandwidth remaining percent 80
    queue-buffers ratio 80

Any hints?

Regards

Alex

[Jon Marshall]

Alex

It's been a while since I did QOS on switches, so this may not help, but it is using CS5 or DSCP 40 because that is the default setting the in the CoS-to-DSCP map on the switch.

You can change this so that CoS 5 maps to DSCP 46 ie. EF.

See this link -

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/3se/qos/configuration_guide/b_qos_3se_3850_cg/b_qos_3se_3850_cg_chapter_011.html#ID942

Jon